This invention relates to techniques for reducing parametric failures and improving yield in nano-scale SRAM and other electronic devices, and more particularly to self-compensating—or self-repairing—techniques for reducing parametric failures and improving production yield in such devices.
Technology scaling has increased and continues to increase the performance of semiconductor devices, but, with increased scaling, process variations including inter-die and intra-die variations become a significant problem, resulting in significantly more failures and lower yield. The success of technology scaling in the semiconductor industry, particularly for microprocessors, strongly depends on the ability to design large and robust on-chip memories, e.g., SRAM arrays, and so there is a need for improved techniques for reducing parametric failures and improving yield.
As discussed in more detail below, die-to-die and within-die variations in process parameters result in mismatch in the strengths of different transistors in an SRAM cell, one of which is depicted schematically in FIG. 1, resulting in functional failures (read, write, access and hold failures). The functional failures due to parametric variations (hereafter referred to as parametric failures) degrade the memory yield (i.e., the number of non-faulty chips). Due to small geometry of the cell transistors, the principal reason for parametric failures is the within-die or intra-die variation in threshold voltage Vt of the cell transistors due to random dopant fluctuations (RDF). The die-to-die variation in process parameters (say, Vt) also has a strong impact on the failure probability of a cell. In particular, low-Vt dies have a higher probability of read and hold failures while high-Vt dies suffer mostly from access and write failures. Thus die-to-die variations significantly increase the yield degradation.
Redundancy is an effective way to reduce the failure probability but it naturally tends to offset the gains achieved from scaling. Thus, other ways are needed to improve yield with nano-scale devices.